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Is there a way for an aircraft to tell a ground station when the gear is down?
Is there any way that someone on the ground can be informed if the landing gear is down? It could be an ACARS message sent to the ground center, or a specific light (sequence) from the aircraft that someone can see visually, or something else. If there is nothing today, is there something that could work in future for all aircraft?

A decent pair of Binoculars is all you need

Are you asking if a) the aircraft can tell someone on the ground when the gear is down, or b) someone on the ground can tell the aircraft when the gear is down?

Both possibilities, if ACARS is already sending this data to the ground center or any other parameter (already being sent) in ACARS that can indicate that state. If that is not possible then am trying to find other solution where if a specific light or any other unique identification that can be used. May be a specific flashing light or anything else. The aim is to find a way to identify this event from the ground , either via ACARS or visual indication.Thanks

Based on your comment, I edited your question to make it a little clearer but if I got it wrong please just roll back the edit or edit it again yourself.

@Ramin What's wrong with radio?

What purpose would there be for the ground to know whether the gear is down or not?

Who exactly do you need to tell that the gear is down? Just a random bystander?

Researching on automating data collection for parameters and scenario defined above .

What would someone on the ground do with the information, besides possibly inform the pilot?

AFAIK, ACARS reports the block time and flight time already. That consists of parking brake release and parking brake set (block time), and weight off wheels to weight on wheels (flight time). These are to help airline maintenance keep track of when they need to do inspections, etc. It would certainly be possible to program it to report wheels up and down, but currently there's no reason to have it report those times.
I'm not sure if you're talking about having this reported in real time or not, but automatic ACARS reports are not exactly in real time. They can transmit things right away (such as text messages from the crew) as long as they have a network connection. But the automated messages are queued for transmission as a block and some types of messages are prioritized over others.
If you're talking about just collecting this data for later analysis, many aircraft are equipped with a quick access recorder (QAR) which records numerous flight parameters. I'm fairly sure wheels up and down times are usually collected. QARs are intended to give information about the flight that can be collected after a flight lands, either by USB, cellular or removable memory cards/disks/tapes. These are collected periodically, such as when the plane returns to the airline hub. This data is used by airlines as part of their flight data monitoring program which looks for incidents that exceed defined thresholds or trends that are occurring to help identify possible safety risks. So the airlines already have a method to collect gear position data.

It is technologically possible to do, and could be retrofitted to all airplanes of all sizes with retractable gear, although you'd need to also fit the system to fixed gear airplanes as well so people on the ground could rest easy in the knowledge that the gear on those airplanes will always be down. The system could be lights, or a radio based system, satellite based, or other technologies.
This system does not exist because the benefits of the system aren't worth the cost of implementing it. It would cost billions to implement the technology for no improvement in safety or operations.

Thanks, my research is about probably finding some solution where there is no change to planes or airlines. Just have to figure out a way where some existing ACARS message can be used or some other external system on the ground can detect that event. Just have to track that event. I really hope and best case scenario would be that there is some ACARS message that is already been sent that can detect this event.

I think you need to get a bit clearer about what your expectations should be. There is no easy or cheap solution to what you are talking about.

Thanks, My first task would be to find a ACARS message that is already being sent while on Approach and if that message has some indication in it if the gear is down or up.There would be so much information that would be passed on from the Plane to ground. Just wanted to know if any message/parameter sent has that info.


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Could an aircraft like a Boeing 777 take off with flaps fully deployed?
I have wondered this for years.  Could an aircraft successfully take off with the flaps fully extended to the landing position? To make this more concrete, let's say a B773-ER at ATL runway 8R.

I don't know about the 777, but on 727-100 and 747-100/200 aircraft, the moment you advanced the thrust levers, the takeoff warning horn would go off with the flaps in a landing position rather than the takeoff position. It's loud and hard to ignore, and sim training constantly reinforces the response that when you hear that sound, you immediately bring the thrust levers all the way back.

The answer is yes it can, the B773ER regularly takes off at flap 20 setting and can performed a go-around at Flap 30 (although the procedure is to raise the flap to 20 immediately).
Taking of at Full Flap (Flap 30) is well within its capability, depending on how heavily it is loaded, how high the airfield is, how long the runway is, how hot the temperature is and how steep the required climb gradient is given the surrounding terrain.
There would be a warning sounding in the flight deck during the takeoff, as this is not a normal takeoff configuration. When the aircraft accelerates above 180kts in the climb out the flaps would automatically retract a stage to protect themselves against the aerodynamic loads

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Is 1N2 (Spadaro) in class G or D airspace?
I am confused about the situation where an airport is located in class G airspace and at the same time located in class D airspace. As we can see the picture below Spadaro (1N2) is located inside the Francis S Gabreski (KFOK), am I right? If so, do I need to call tower before entering class D?

You have two different questions here so I edited it to focus on the first one: what airspace is 1N2 in? I don't think we have a question on this site about the requirements for entering class D airspace (I may be wrong), but the answer is fairly well known (91.129 and AIM 3-2-5). You can always ask that as a new question if you like.

Strictly speaking class D airspace generally has a radius of 4 NM. If you take a look at the TAC chart or hop on your favorite flight app you will see the field actually sits just outside the airspace.

The sectional is likely drawn this way to allow for 49N and 1N2 to both be properly depicted.
Officially the airport is closed according to the current AF/D it appears to have been shut down in 2016, so it may be listed there simply for visual reference as the physical runway seems to still exist but traffic overlap is no longer an issue.

Thanks for answering the question.

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What is the required cloud clearance in G airspace when there are clouds just above in class E airspace?
This is my first question here and I am about to start my Private Pilot License (PPL).
Now I am flying in my Flight Simulator X. So, I am at KDKK airport. It has an elevation of 692 MSL . I got my ATIS report 2100 MSL scattered and 3000 MSL broken. The airport is in class G and the weather minimum here is clear of clouds.
At 1200 AGL we have class E airspace and 500 feet below clouds rule applies here. The question is: flying at 1800 I will still be flying in G airspace, but will not comply with 500 feet class e rule and will be closer than 500 feet to the clouds. Is it legal?
And the second question: still didn't find clear definition of "clear of clouds". Is that mean that there are no clouds at all? or it could be clouds, and I can fly very close to them, but I can't get through the clouds?

I think I made a mistake in my question when gave ATIS altitudes in MSL.

Related

Yes, as in your question here you do not account for the fact that the ASOS gives the ceiling in feet AGL. But the question is still relevant if you consider the case when ceilings are 700 feet lower than what you said, i.e. actually 2100/3000 MSL.

KDKK is under a Class E shelf starting at 700 ft AGL (Magenta vignette around the airport).  If the airport elevation starts at 692’ MSL, the Class E shelf will begin at 1392 ft MSL.  Therefore operation at 1800 ft MSL above KDKK will put you in Class E airspace and subject to the VFR weather minimums of 3 sm visibility and 1000 ft above / 500 ft below / 2000 ft lateral cloud clearances.
As the the second question, clear of clouds means remain in a position outside of clouds and a flightpath which will not place you in immediate risk of entering a cloud.

Sorry, it was my mistake. BUT what if there is no magenta vignette around the airport and class E starts at 1892 ft? Just want to understand cloud clearance in my scenario in my question. Thanks.

Then if you’re flying at 1800 ft MSL, you’re in Class G and subject to Class G VFR weather minimums.

Thanks. So, I can fly in class G, just right below class E, even if there are clouds in class E and clearance between me and clouds less than 500 ft.

It’s not the wisest idea, but it is legal.

You might want to add your "not the wisest idea but it is legal" comment to your answer: that seems to the key piece of information that the OP was looking for.

@CarloFelicione, there's a typo clear "or" clouds.  (it's not possible to make short edits to repair typos)

Probably not worth an edit to such an old question especially as comments indicate question didn't really express what asker really wanted to know, but "Class E floor" might be clearer than "Class E shelf", as "shelf" seems to imply that the unit of airspace has a top limit too, and (unless we want to look all the way up to the Class A floor way up at 18,000'), most pilots wouldn't think of the Class E as a "shelf".  Not technically wrong (because there is an upper limit to the Class E, at the Class A floor), but just a thought.

WELCOME to Aviation Stack Exchange!
Carlo Felicione gave you a great answer. But to help you along, I keep a printed copy of page 1-3 from FAA-H-8083-15B-- instrument_flying_handbook.pdf and a copy of the sectional legend in my aircraft's slip pocket for quick reference. A quick glance lets you visualize just about any common airspace that you may fly into. I had a friend with a high quality color laser print them front and back and then laminated it.
A visual aid helps answer questions such as yours better than my memory. I also use MS FS flight simulator to keep my skills during the winter so I also keep a copy of these next to my computer.

Not sure what to do here. This does not (event attempt to) answer the question, but provides extremely useful advice. It should be a comment, but comments are ephemeral, and as I said, it is very useful advice to a new pilot in training.

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What is the meaning of the percentage used to describe the thinness of this glider's airfoil?
In a glider review for the Schempp-Hirth Discus, I encountered the following paragraph:

The Discus 2 airfoil is thin (about 14.5 percent), incorporating studies by K.H. Horstmann and Dr. Wuerz (wing) and Luc Boermans (tail).

After researching this a bit, I've been unable to find an explanation for what "14.5 percent" was referring to.
I thought perhaps that it is the ratio (14.5/100) of the thickness of the wing to the width of the wing, but that is really just an educated guess.
The Wikipedia page on airfoils has a section entitled Thin airfoil theory, with two bullets which also might qualify:

(1) on a symmetric airfoil, the center of pressure and aerodynamic center lies exactly one quarter of the chord behind the leading edge
  (2) on a cambered airfoil, the aerodynamic center lies exactly one quarter of the chord behind the leading edge

Is this "one quarter" the ratio that is mentioned in the article?
I did see this post, but had trouble seeing how the 90% and 99% numbers mentioned in the answer are related.

14.5% is a relative thickness (thickness divided by chord)

You could call it the 'cross-sectional aspect ratio'.

14.5% given in the article is the (maximum) thickness to chord ratio, expressed in percentage. It is usually used to explain how thin (or thick) the airfoil is. Note that this is a characteristic of the airfoil.
From the NASA page Wing geometry definitions:

Airfoil thickness to chord ratio, image from NASA page Wing geometry definitions

... The straight line drawn from the leading to trailing edges of the airfoil is called the chord line. ... The maximum distance between the two lines is called the camber, which is a measure of the curvature of the airfoil (high camber means high curvature).
The maximum distance between the upper and lower surfaces is called the thickness. Often you will see these values divided by the chord length to produce a non-dimensional or "percent" type of number.

Well, at least the answer was on the page "intended for college, high school, or middle school students" :-).  Seriously, the linked page does a great job of putting this stuff into layman's language.  Thanks for the answer!

From this link:

The red line is the cross section of a wing. The main dimension in the profile is the black line in the middle at a slight angle: the wing chord. All dimensions in this NACA profile are defined relative to the wing chord:

2% camber (the slight upward bend in the middle line)
30% max camber position: top of the bend occurs at 40% chord
12% thickness: maximum thickness of the profile is 12% chord.

The profile is defined this way because for instance with a tapered wing planform, we can still define the profile at any point at the wing if defined relative to chord.

Clarification?  Is it the black line that is the chord?  The black line looks to me like it is part of the graph and doesn't look like it curves.  Wouldn't it be the gray line?

@bclarkreston, yes, the black line is the chord. It is also the $x$ axis, because the profile is normalized to have the leading edge at origin and the trailing edge at $(1, 0)$. The gray line is the camber line.

Got it...that made sense after reading the NASA page @aeroalias posted in his answer.


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What is this abandoned airplane parked in Tegel airport?
This airplane is parked in Tegel airport. Probably something extremely common, but can you please help to identify it?

Related: What is this aircraft with large pipes at the flight deck windows?

Looks a whole lot like a Boeing 707

Thanks, it apparently is a 707-458 and has an interesting history https://digitalcosmonaut.com/2016/berlin-tegel-abandoned-boeing-707/ .

Like most of the abandoned aircrafts. In some countries, learning the story of an abandoned aircraft can tell you a lot for the country itself, especially when the owner was the country itself.

People normally include pictures or further reading links in aircraft identification questions.

In the far corner of Tegel Airport, to the southwest (here a satellite image), lonely a plane with a special story rots: The Boeing 707 was a gift from the manufacturer to the Lufthansa and landed in 1986 by tricky preparations as the first Lufthansa aircraft in West -Berlin. Otherwise only Allied aircraft could fly through the east corridor. Lufthansa donated the plane to the Berlin Technikmuseum, which, however, can not do anything with it. Every now and then, the airport fire brigade on the Boeing exercises in case of emergency.

https://www.tagesspiegel.de/mediacenter/fotostrecken/berlin/flughafen-tegel-das-vergessene-flugzeug/13531996.html

.. poor thing :-(

I wonder if a Kickstarter or other fund could be started to restore it to its former glory.  See if Lufthansa wants to contribute.

Unlikely, as they are not willing to fund the Constellation project: https://www.nzz.ch/mobilitaet/luftfahrt/lufthansa-oldie-projekt-super-constellation-im-sinkflug-ld.1367157 (in German)

It looks like a 707-458, due to its four engines on its wings.


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Why are "canard" not used on commercial aircraft?
As of today and as far as I know, there is no commercial aircraft with canard, while they are used on several military aircrafts (like Rafale, Typhoon).
Plus, during take off the empennage pushing the rear of the aircraft down to pitch up add pressure on wheels, and this was for example one big design flaw on the Concorde:

Because of the way Concorde's delta-wing generated lift, the undercarriage had to be unusually strong. [...] Combined with the high airspeed at rotation (199 knots indicated airspeed), this increased the stresses on the main undercarriage in a way that was initially unexpected during the development and required a major redesign..

I feel like addind a mobile canard in front of the plane would help to rise the nose up to take off.
Why are "canards" not used on commercial aircraft?
Edit about possible duplicate: I do not feel it is a duplicate of this question because "my" plane would be a three-surface aircraft with a canard, a main wing (not delta but classic) and conventional tail-plane. The canard would be mobile and add pitch control.

From my understanding, the traditional design of having the elevator and vertical stabilizer behind the main wing causes the aircraft to be more stable around the yaw axis. The farther back it is, the more stable it will be. The farther forward, the less stable. But one thing to consider is that having it fore of the wing will cause the plane to have better manoeuvrability at the expense of less stability.

"addind a mobile canard in front of the plane would help to rise the nose up" Getting the nose up usually isn't the problem. The aircraft isn't going to take off until the wings generate enough lift to counter the weight of the aircraft. Getting the nose up too soon is a good way to stall the aircraft in ground effect, or lift off in ground effect and stall in the climb.

Well the Tu-144 was one example of an airliner which made use of canards.  The design configuration has been proposed before for large transports such as Boeing's ill-fated Sonic Cruiser.

And I suppose if you consider other, smaller aircraft, like the Paiggio P.180 Avanti for use in Part 121 and Part 135 operations, the that configuration is used often.
Probably the biggest reason is there hasn't been a pressing need to do so or the costs to abandon all of the engineering knowledge base in favor of a brand new design does not outweigh its potential benefits.

I'd go further and take one of the reasons the Beech Starship failed: many potential buyers are far more conservative when it comes to technology than designers and don't want things that are radically different from what they're used to seeing.


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Were cargo containers on the OV-1 Mohawk ever used, and if so where?
According to some specifications (e.g. the OV-1B Standard Aircraft Characteristics), the OV-1 Mohawk could carry cargo containers for the emergency resupply of ground forces. I failed to find more information about these containers or any pictures.
Where the supply containers ever actually developed, issued, and used operationally? If so, what models and where? Were there different containers for airdrop and airlanding?
(I realize these are several questions, but I believe they belong together. The perfect answer would be a link to photos, container designations, and a description, along with units and dates. But any one part might help with further search.)

There is evidence to suggest that cargo pods were developed to be dropped from wing-mount hardpoints for the purpose of emergency resupply. Whether they were ever issued or used operationally is not (to my eyes) possible to establish.
At issue however: Cargo pods, and especially wing-mounted pods must be designed to minimize the aerodynamic impact on the aircraft's performance.  As a consequence, their visual appearance is rarely distinct.  Consider the example, below, mounted to a F-16:

While at this point I'm speculating (I'll update with official docs if I find them), the likely candidates for cargo pod development would have been the Mark 79 Napalm cannister or a fuel drop tank. Photographs of the OV-1 may, therefore, depict such equipment but you'd never know it because it looks like a fuel tank or napalm bomb.
Some documents refer to the "M4A Supply Container."  M4A is a common file format, and M4 refers to so many pieces of materiel at this point that I've yet to successfully cut through the noise to find documents on that piece of equipment itself.
TL;DR - Likely developed, possible to have been issued and even carried, but good luck proving as much.

100% more information than I had so far. :-) But M4A sounds weird. M5 would be Mark 4 and A is an approved modification, but that should have another number following the A. Or is it /A?

@o.m. The references, probably all pulling from the same source, just say "M4A supply container." Verbatim. Three hours of searching and I haven't found any such equipment to link. I'll update the answer if/when I do but I'm not hopeful.

Actually, when I think about it... it seems plausible they modified an M4 20mm gun pod as a cargo box? The OV-10 carried that one, and that's the OV-1's successor.  I'll dig into the gun pod's history when I get a chance.


text:
Do Federal regulations compel airlines to avoid schedule disruptions?
I understand that airlines may classify certain passengers as "must-ride" and, at least as a matter of policy, bump regular passengers in order to accommodate "must-ride" passengers.  I also understand that it is common for airlines to classify crews riding deadhead as "must-ride" based upon whether or not they are urgently needed at their destination.
I was recently informed that federal regulations actually compel airlines to do everything they can to ensure that crew members successfully make it to their destination in order to crew a scheduled flight, but I was not given the specific regulation that applies.
Is there a DOT or FAA regulation that requires airlines to bump standard passengers in order to transport "must-fly" crew members?  Is "must-fly" status just a matter of airline policy, or is it some FAA- or DOT-defined status that airlines make use of?
If not, is there a more general regulation that compels airlines to avoid cascading cancellations at all costs wherever possible?

Crew members not in time at their departure airport can be replaced anyway.

The Department of Transportation has A Consumer Guide to Air Travel that discusses passenger rights regarding delayed and cancelled flights. Some key points:

Airlines don't guarantee their schedules, and you should realize this when planning your trip.
Overbooking is not illegal, and most airlines overbook their scheduled flights to a certain extent in order to compensate for "no-shows."
Airlines set their own "boarding priorities" -- the order in which they will bump different categories of passengers in an oversale situation.
DOT requires each airline to give all passengers who are bumped involuntarily a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't.

There airline is required to provide various sorts of compensation if they bump you involuntarily and can't get you there within 1 hour of the original flight. This depends on how late you will arrive and what kind of ticket you have.
There is no rule that says airline employees must fly instead of customers, but each airline has their own Contract of Carriage that describes their rules. You can read United's here as an example. Rule 25 describes the process for denied boarding:

If there are not enough volunteers, other Passengers may be denied boarding involuntarily in accordance with UA’s boarding priority:
The priority of all other confirmed passengers may be determined based on a passenger’s fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment.

Of course IANAL but the way I read it, they "may" use any of those methods but they are free to deny boarding as they wish. Nothing stops them from choosing to bump customers before employees. Delaying or canceling a later flight because they bumped the employees would be more expensive than paying a few customers to be bumped involuntarily. But bumping employees may be less expensive than a PR fiasco.
There is also a program called Essential Air Service, where the DOT subsidizes airline service to smaller markets. Airlines bid to provide this service under certain conditions. Although I wouldn't say they are "compelled" to provide that service, if they don't meet the conditions, which could include not completing enough of the scheduled flights, they may not receive the subsidy.

The only information I could find that resembles a regulation is 8000.75, but this order specifically applies to safety inspectors, not to flight crews.
Beyond that, there IS law regarding denied boarding at 14 CFR 250.2a, which states (emphasis added)...

...every carrier shall ensure that the smallest practicable number of persons holding confirmed reserved space on that flight are denied boarding involuntarily."


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In the International Standard Atmosphere/ICAO, what is geometric and what is geopotential altitude?
Read first before you start considering it a duplicate, please, as the similar questions didn't answer my question either.
The usual standard atmospheres have two altitudes: the geopotential and the geometric one and I'm still confused on which is which and which one an aircraft's altimeter is showing.

In the U.S. 1976 Standard Atmosphere (i.e. not the ISA/ICAO but perhaps in theirs too) the geometric altitude is the true altitude above sea level while the geopotential altitude is the one that considers 1g to be at any altitude (i.e. as if gravity didn't decrease with altitude, remaining 1g all the way up). This makes sense as the pressure value is lower for the geopotential value than for the geometric value, implying that gravity pushes down more strongly because it's considered 1g, not a lower gravity, in the geopotential value.
The WP page however, considers the geopotential altitude to be the gravity-adjusted one. Perhaps the ISA defines the altitudes differently, and "geopotential altitude" in the ISA means what "geometric altitude" means to the U.S. SA? This link seems to support that because it says the pressure altitude is the geopotential one, and then goes on to say 262 hPa is the pressure at 33,000 ft, which in the U.S. SA is the geometric altitude value, not the geopotential one (which would be 261 hPa).
The U.S. Standard Atmosphere abbreviates the geometric altitude as "Z" and the geopotential altitude as "H" but I've seen the geopotential altitude be called "Z" too (or was it lowercase "z"?). The WP page calls the geopotential altitude "Z" and the link from above calls it "H" and the geometric altitude "h".

Now, to finally get rid of all confusion, I'd like to ask: when a plane's altimeter shows that it cruises at 33,000 ft (FL330), does that mean it is at 261 hPa or at 262 hPa?

You might consider asking on Earth Science SE.

@MichaelHall Well technically my question is what altitude a plane's altimeter (when calibrated to 29.921 inHg) is showing.

Technically, yes.  I wasn't suggesting that your question was off-topic, only that because of the somewhat arcane nature of the difference between the two you might get a better answer somewhere else.  Certainly there is overlap...

The altimeter is calibrated to the geopotential height.
At a geopotential height of 33000 ft the standard atmosphere model will give, according to ICAO doc 7488, a pressure of 262 hPa. This corresponds to a geometric height of 33052 ft AMSL.
An altimeter calibrated to the standard atmosphere and set to standard pressure (1013.25 hPa) will, when measuring a static pressure of 262hPa indicate FL330 (33000ft)

Very well, thank you! Obviously the U.S. Standard atmosphere confused me. Their "geometric" altitude is in fact the ISA's geopotential altitude then, which, as the WP page is stating, is the gravity-adjusted and therefore correct altitude.


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How exactly does the cabin pressure of an airliner change during ascent and descent?
At cruise altitude, airliners' cabin altitudes are usually at 6000-8000 ft, but how and when does it get there respectively how does it behave from ground until cruise altitude?
Is the cabin unpressurized until the plane is at 8000 ft and from there on it maintains the 8000-ft-cabin altitude while further ascending? Or does the cabin altitude change all the time just much more slowly than the actual altitude so that the 8000-ft-cabin altitude is reached only at the plane's cruise altitude?
It's a long time I flew on an airliner the last time so I don't remember properly when my ears started/ceased to feel it but I think they were registering changing pressure when the airplane was above 8000' as well.

Related, but not an answer-- https://aviation.stackexchange.com/questions/11456/how-does-an-airliner-deal-with-cabin-altitude-at-an-airport-above-8000-ft/11463#11463

@quietflyer The answer would rather speak in favor of my former option (unpressurized until at 8,000') but I believe to remember that my ears felt a pressure change at considerably higher altitudes.

@quietflyer Do you recall when your ears felt the pressure change? I haven't flown on an airliner for more than four years.

@quietflyer Yes, thank you. I only replaced the "still" by "as well" because I mostly believe to remember it from descents rather than ascents.

I’m positive there are other questions here that answer this fully.  I even remember graphs….

Highly related (possible answer?) - apparently there is a need to make sure the cabin gets to below the airport altitude before touchdown-- https://aviation.stackexchange.com/questions/26283/how-does-the-cabin-pressure-controller-work-in-a-large-commercial-airliner/26345#26345  -- still unanswered seems to be, in the climb, is the duration of change of cabin altitude often, or sometimes, longer than the duration of change of airliner altitude --

The pressure in the cabin changes proportionally according to the height so that when the aircraft is at cruising height the inside pressure reaches the value you wrote

@sophit -- assuming that cabin altitude is not desired to exceed some given value, evidently pilot must plan ahead and preset intended cruise alt into system?  So system knows what ratio to maintain, to end up reaching target cabin altitude exactly as cruise alt is reached?  Is that really how it is done?  And esp on initial climbout with a lightly loaded plane, might cabin elev sometimes increase too fast for passenger comfort with that system?  Hope we get a good answer that addresses all these points --

Yes, outside pressure at cruising speed is know before taking off (from other airplanes, ISA, measurements, ...). The adjustment during climb and descent is obviously done automatically. The adjustment law is more or less linear. One-way valves prevent under and overpressure.

@quietflyer and sophit Well then I guess that most airliners have a cabin altitude of 8,000 ft at 40, 41 or 43,000 ft. If they cruise at a lower altitude, the cabin pressure is higher then.

Another one with a graphic: https://aviation.stackexchange.com/a/16799/3159

8,000’ is the maximum cabin pressure altitude allowed, and will usually only happen if the aircraft is at it’s maximum altitude, and the maximum pressure differential for that aircraft. Airliners don’t often cruise at their maximum certified  altitude. Many long haul airliners are unable to climb higher than about 29,000’ feet for the first few hours until sufficient fuel is burned off. In those cases the cabin pressure altitude will start off much lower, and then slowly climb each time the aircraft climbs.
In general, most airliners at typical cruising altitudes will have a cabin pressure altitude of about 4,000’ to 6,000’.
There are many different design philosophies regarding aircraft cabin pressurization systems. Some are fully manual, some are fully automated, and lots in between.
In all cases the cabin pressure altitude is designed to be slowly raised as the aircraft climbs, and to slowly descend as the aircraft descends.
As an example, an aircraft may take 10 minutes to climb from sea level to 30,000 ft at 3,000’ per minute, while the cabin climbs to 4,000’ in 10 minutes at 400’ per minute.
If there is a system fault, or pilot operating error, the cabin pressure altitude can sometimes climb or descend at the same rate as the aircraft altitude.

Your last sentence describes Helios 522 and that scenario is something to be avoided. Helios 522 had the same warning signal for two different events, and the pilots confused them.

Cabin pressure is typically regulated from the moment power is set for takeoff
(on the runway), where cabin altitude goes slightly below the runway elevation. As the aircraft climbs, the cabin altitude is climbed at a proportional rate respecting passenger comfort and aircraft design limits.
On the older aircraft types, a pilot or flight engineer would set the planned cruise altitude during the preflight setup. When the plane gets to that altitude, pressurization would go into cruise mode, keeping the altitude steady even if the aircraft makes slight moves above and below the associated flight (pressure) altitude. On newer aircraft, the pressurization system would get the planned cruise altitude from the flight management computer.
Before descent, on older planes, the landing airport elevation would need to be set. As you may expect, modern planes would get landing field elevation from the FMC.
The pressurization system would recognize movement away from the cruising level, and if the movement is a descent, would switch into a mode proportionally reducing the cabin altitude as the aircraft makes its way down to land. Ideally, the cabin altitude at touchdown would be slightly below the runway elevation, then gradually equalize during the landing rollout.
This is a simplified version of airliner pressurization. Things can become more complicated, as when the flight never arrives at the planned cruise altitude or diverts to a field with a different elevation from the planned destination.
(grin) I once flew a turboprop with overpressure protection based on spring loaded outflow valves. If you climbed above the altitude corresponding to the max internal / external differential, cabin pressure would crudely push open those spring loaded valves and the result was wild "pressure bumps."  Instead of a nice 500 to 1000 fpm change in cabin alt, it would be a sudden and uncomfortable 1500 fpm or more.

I have also flown in aircraft with spring loaded overpressure valves and I know of at least once case of the valves sticking for the first 20,000 ft of an ascent and then explosively decompressing the aircraft to the valve closure altitude of 7000 ft. This caused barotrauma to a patient aboard and sinus pain to the medical attendants.

@gtothill Whoah, that is a bad circumstance, to have the outflow valve(s) stick until such a high pressure differential! One would expect a reasonable maintenance regimen to include some sort of lubrication or exercise of the valve in order to prevent that. In my answer, I didn't mention the effect of passengers smoking tobacco, but the old salts used to speak of outflow valves being fouled by tar - until airlines went smoke-free.

What's the reason for slightly over-pressurizing the aircraft at takeoff and landing (i.e., cabin altitude below runway elevation)?


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How would a wheel-well stowaway have performed on a Concorde?
There have been many news reports of stowaways who hide in the wheel wells of subsonic airliners and survive the flight.
I assume this would not be survivable with a Concorde because they'd be unable to breathe above ca. 50,000 ft [citation needed].  Therefore, would their death have been certain?  Would a human even fit into the Concorde's wheel well?  Are there any known examples of attempts?
I wonder what exactly would happen to their body over the course of the Concorde's cruise, as opposed to a normal airliner stowaway. What effects would the supersonic cruise have on their bodies when in the unpressurized wheel well?

Comments are not for extended discussion; this conversation has been moved to chat.

Dead.
Let's presume a person could fit themselves into the wheel well, and we now wish to establish an objective survival time, say, 90 minutes, for the flight at altitude.  The Concorde altitude at cruise is 18300 m, or 60000 ft. A long out-of-print insert by McDonnell Douglas Astronautics Company, that was in my aeronautics and astronautics text, has the following somewhat cautionary statement:

Blood at normal body temperature (98.6$^\circ$F), boils at this pressure altitude (63000 ft).

Quite obviously, the air density is so low that insufficient oxygen is present for survival.  Even with a pressure mask, and oxygen, the experience outside of the aircraft would be harrowing.

Blood doesn't boil at altitude because the positive blood pressure.

And even if the person'd be wearing an oxygen mask he'd be dead from exposure to the very low temperature up there.

@Sanchises wrong, Depends on the altitude whether the positive pressure can compensate for the very low pressure outside. At the temperatures we're talking about it can't and would fill the lungs, causing you to drown.

@jwenting Even in the vacuum of space, blood does not boil as long as it's inside of your body. Water boils at 47mmHg@37°C, diastolic pressure is much higher. Nitrogen may form bubbles in the bloodstream, and indeed water may travel through thin membranes like your lungs and boil off there.

@Sanchises it won't remain in your body as it'll diffuse into the lungs. Don't think the pressure will be low enough to have it diffuse through the skin.

That we can agree on.


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Why is there a helicopter visibility reduction restriction for this approach?

For the RNAV (GPS) RWY 17 approach at KSMN, why does it say "Helicopter visibility reduction below 3/4 SM NA."?
Looking at the LNAV, there is a 5 mile visibility requirement and if we cut that in half per AIM 10-1-2, wouldn't there be a 2 1/2 mile visibility requirement? If so, then a helicopter couldn't fly the approach with 3/4 mile visibility anyway.

I'm not entirely sure what your question is.

@CarloFelicione -- his question's fairly simple actually: "why did the FAA put a note on the plate that has no effect in practice?"

It does seem odd. My only thought is that perhaps some §135 operations could have authorizations that could allow them to use lower visibility minimums, apart from a prohibition like this.

Can't an helicopter approach on a 350° course and then circle to land to RWY 17. In this hypothetical explanation, the 5 mile requirement wouldn't apply.

@mins The circling minimums are the same as the LNAV ones in this case, so you would still need 5 miles visibility to circle to 17

AFAIK, the 3/4 SM restriction is there because of obstacles (presumably the tower between the MAP and the runway) but I have no idea why they printed it when the minimums are much higher anyway. My guess is it was included by default but the best way to get an answer might be to ask the FAA. We've done that before :-)

Yes, @UnrecognizedFallingObject is correct, that was my question. My only guess could be to restrict a VFR visibility reduction from 1 SM to 1/2 SM in Gulf Airspace at night per 91.155(b).

Helicopters may operate under special VFR when in flight visibility is less than 1 SM.  Therefore, the approach can be flown at lower visibility than that published on the approach plate.  However, the note must be published to further restrict these types of operations:  "The procedure is annotated with 'Visibility Reduction by Helicopters NA.' This annotation means that there are penetrations of the final approach obstacle identification surface (OIS) and that the 14 CFR Section 97.3 visibility reduction rule does not apply and you must take precaution to avoid any obstacles in the visual segment."1

Remember that the helicopter visibility restriction also applies to IFR departures.  under part 135, you can depart IFR without a takeoff alternate if you have the visibility to shoot the approach straight in, and all the equipment is working... this visibility can be cut in half unless it says helicopter vis reduction N/A.
So, looking at this approach plate alone would still not answer your question.  But, if you look at the other VOR/DME B approach you can see that the required vis is 1.25sm vis.  Helicopters would usually be allowed to cut that in half and therefore depart IFR without a takeoff alternate if the vis was .625sm vis.
I looked it up, but they seem to have removed the limitation as of me writing this.  But I would be curious if they had that limitation on all of Lemhi's approaches in order to limit the IFR departure to 3/4sm.

Welcome to Aviation.SE!  Could you provide some sources for your answer?


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Where can I get airport diagrams of non-towered airports?
I'm trying to find a couple of airport diagrams for right now: X26 and X59.
Neither of them are listed in the Chart Supplement or on the FAA site.
All I could get were those shabby diagrams presented by Airnav and Skyvector and sorts, and Google Earth images, both of which are not current. (The runway numbers are different from what are actually designated (painted) and used.)
Does anyone have a better idea about where to find the airport diagrams?
Plus, what the heck is the order of those airport diagrams listed in the Chart Supplement?! I couldn't find any rules.

What are the current runway numbers for X26? I'm finding a diagram in the d-CS just fine...

@UnrecognizedFallingObject Looks like they're 10/28 and 5/23 like in Carlos post

Many non-towered airports have diagrams in the Chart Supplement, and there is an airport diagram for X26 but not for X59. The difference appears to be that X26 has instrument approaches but X59 doesn't. It looks to me like the airport diagrams are considered part of the terminal procedures.
But I have to say that the FAA is as clear as mud on this. The airport diagrams legend says:

selected towered airport diagrams have been published in the Airport
  Diagram section of the A/FD

But obviously that's completely wrong, because a) many non-towered airport diagrams are in there, and b) the A/FD doesn't exist any more.
By inspection, it looks to me like airports without an instrument approach have no airport diagram (X59, X25, 2RR) and ones with an approach do have one (X26, X07, AVO). But I only spent a few minutes looking, so I may well be completely wrong on this.
As for your question about the order of diagrams, the legend says:

Diagrams will be listed alphabetically by associated city and airport
  name

No objection to your insight about the "selected airports" so far. Thanks!

And thanks for pointing that out as well. I was skippy through that header in the Chart Supplement. That was, however, my initial thought that it seems like they're in the order of those names of cities or of airports, and then I just gave up on it when I found that there are SOME exceptions in that order. Seems like these ones had their airport names changed recently but still some others are not in the order by the name of the city.
To sum it up for the order: most of them are sorted out by city name, but some are by airport name and the latter ones' names' have been changed.

Does anyone have a better idea about where to find the airport diagrams?

For U.S. airports, consider using the AOPA Airport Directory. It's searchable by anyone without charge. I just checked, searching using X26 as the input string and then X59, and it has airport diagrams for both.

Try contacting the airport manager at those airports for this or more info:
X59: 321-952-4590
X26: 772-228-7013
ForeFlight makes a basic airport diagram for X26 and for X59.

Interesting that they have one for X59 -- X26 has one in the Chart Supplement, but X59 doesn't for some odd reason

Thank you!! I was curious in general regarding those non-towered airports but anyways.. I guess I have to get the ForeFlight first. Or try contacting them.


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Can a flight attendant have only one eye?
I have only had one eye for many years and was a flight attendant before it happened now have received a one on one interview with airline. I just wonder if my eye will be a problem.

So you're saying you're going to be a sky pirate?  I think that's pretty awesome, would hire.

@user23206 how did the interview go? If possible please update.

https://www.youtube.com/watch?v=L2W7r_aNxrg

Let me refer you to the answer of this other question, that says a pilot may be considered for any class of medical (including First Class) with only one functional eye.
I think its fair to conclude that if a pilot can get a First Class medical with one functional eye, then you can be a flight attendant with one functional eye, assuming that you can fulfill all of the duties required.
Good luck.

@camden_kid yes, click on the question that I linked in my answer and read about a famous one-eyed pilot.

As far as I know, there's no FAA requirement for flight attendants to have a medical. There is a requirement in 49 USC 44728 to hold a "certificate of demonstrated proficiency", but that's it.
I couldn't find any clear details on what the certificate requires, but it looks like each operator (airline) creates their own certification program and the FAA approves it. Based on the name, I assume that it's all about whether or not you can do the job rather than any specific medical requirements.


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What are "secondary precessional forces" that apply to boomerangs in flight?
note: This question is about the aerodynamics of boomerang flight and how it differs in microgravity environments. Some background information available here in Aviation SE: Are frisbees and boomerangs technically gliders? and in Space SE: How does one throw a boomerang in space? Does it return?

Professor and former astronaut Takao Doi is apparently the fourth person or the third person to throw a boomerang in space and the first to throw one specifically designed to work in microgravity.
The Space-Toys.com article says:

Finally, on March 18 2008, Japanese astronaut Takao Doi took a three bladed boomerang (supplied by Yasuhiro Togai) aboard the Space Shuttle Endeavor on a mission to install equipment on the International Space Station. The boomerang that he threw was the only space boomerang that was specifically designed for space flight. The photo of the boomerang that Maurizio supplied to me show that the blades are recessed to the rear, as you would find in a Fast Catch boomerang. This is the optimal design for a space boomerang because when the blades are shifted to the rear, the secondary precessional forces that cause a boomerang to lay over in flight are retarded. The Japanese team of Yasuhiro Togai and Takao Doi deserve credit for making and throwing the first [multibladed] boomerang that was optimized for use in microgravity conditions.

What exactly are the "secondary precessional forces" referred to in the article?

Takao Doi in a space three-point stance throwing a returning boomerang aboard the ISS:

Not sure this question will survive here, the topic being pretty borderline as aviation related.  You'll probably get sent to SE Space or whatever it's called.

@JohnK for a question about the aerodynamics of low velocity flight and secondary precessional forces therein I'm fairly certain Space SE would not accept a proposed migration. I'm pretty active there and for this specific question I think the best bet for topicality is here.

I edited your question a little; I removed the supernovae information because it didn't seem to be relevant here but of course if I changed too much then you can always roll back or edit again.

@Pondlife Thanks, I'll trust your judgement; I'm a visitor to these lands... :-)

or [physics.se], they're a laugh-a-minute crowd...

@CGCampbell I hear ya. I have heard it said that Physics SE is "broken". It's better than it used to be, but I still tend to avoid it.

The aerodynamic forces on the boomerang are roughly like the springs that limit the (primary) precession of a rate gyroscope.  These forces introduce a secondary torque, which in turn produces the secondary precession in question, which is needed to tilt aka "lay over" the boomerang.  (My physics is rusty; the physics SE may be more fruitful.)


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What may be the cause of hot start problems in a Lycoming fuel injected engine?
Cherokee Six, 32-300 1969. Lycoming engine is fuel injected with 450 SMOH. Cold starts are easy: 1/2" throttle, full rich, fuel pump on and hit starter after fuel flow starts. Runs great once running.
Hot starts are hit or miss. If it sits over an hour it may start but if I miss it it has to sit for at least another hour till I try again or forget it. I've tried 1/2" throttle, full lean with and without fuel pump; full throttle, full lean with and without fuel pump and all combinations I can think of. Most of the time it spins without any cough or catch. Due for 50 hour oil change and will ask that plugs and magnetos are checked. I'm afraid to go anywhere for fear of being stranded for hours.

Have you asked your A&P about this?

You might try reading this article: http://flighttraining.aopa.org/students/presolo/skills/hotengine.html

Troubleshooting a no-start situation like this is probably too difficult for a simple Q&A site - about the best advice we can offer is that you need fuel, air, and spark. You likely have no problem with spark (because you can start the engine when cold), so you'll need to work with your A&P to troubleshoot fuel (vapor lock?) and air - try to get the plane to exhibit the problem for your mechanic. Also make sure you're trying to start the engine in accordance with your POH or the Lycoming operator's manual - the published techniques generally work unless something is wrong.

For what it's worth I also had very difficult hot starts with a Lycoming IO-540 (I assume that's the engine you're talking about). Had to be very careful not to flood it and I often wondered whether vapor lock was an issue. Never solved that mystery.

@dvnrrs Vapor Lock is absolutely the issue.  The fuel injection plumbing is on the top of the engine which is the cold side of the engine when you are flying but when parked, heat rises!  The accepted answer below cures this by running cooler fuel through the plumbing while flooding the engine, then the flooded start procedure is necessary.

@SkipMiller That makes a lot of sense. I don't fly the IO-540 anymore but this (and the suggested remedy) is great knowledge for the future. Thank you!

The issue is undoubtedly vapor lock. In the fuel injected Lyc 540, the fuel injection plumbing is on the top of the engine which is the cold side of the engine when you are flying but when parked, heat rises! The following procedure runs cooler fuel through the plumbing while flooding the engine, then the flooded start procedure is necessary.
First, check your POH for the hot start procedure for your plane.  If not there, try this as it is for a very similar airplane:
Hot Starts on the Saratoga PA-32-300:

Intentionally flood the engine.  This runs cool fuel through the plumbing and "breaks" the vapor lock.

a) Full rich
b) Wide open throttle
c) Fuel pump on, watch fuel flow gauge
d) When gauge registers flow, wait 3-5 seconds then red knob to ICO and pump off.

Crank engine with red knob still at ICO and with throttle wide open at first. Gradually close throttle (at a rate that will take you about 10 seconds to close the throttle) and it should start up at some point.
When engine catches, do several things more or less simultaneously:

a) release ignition key
b) throttle to idle to avoid over-revving
c) mixture full rich.

Tried flooding it and no excessive fuel spilling on ground going along with vapor lock and fuel starvation. 2 lower plugs with high resistance too were replaced but mechanic didn't think that was the culprit. Haven't had a chance to try a hot start since all the suggestions. Also no fuel return line in Lycoming per mechanic so just running electric pump with mixture ICO for 1 minute unlikely to flush cold fuel in the lines but will try anyway next unsuccessful hit start.

Right, no fuel return line.  It pushes the fuel/fuel vapor into the intake manifold flooding it.  The procedure I posted is not the flooded start method in the POH but it was taught to me by my CFI and it worked for me.  Still a bit of an art, though.  Keep us posted.

@Marshall: I just re-read your comment.  Note that the procedure calls for full rich so you are not running the pump with mixture at ICO.  And please note that one minute is waaay too long to run the pump in this case.  The fuel flow gauge doesn't move for a while as the pump is pushing mostly vapor through the system.  When the gauge starts to indicate some flow, then 5 seconds, maybe 7 max and then back to ICO and pump off.

No need to run the fuel pump as @Skip Miller says.  (though it may say that in the Saratoga manual)
I have done the following with success for hot starts on a IO-320, IO-360, and IO-540.

Throttle Full Open. (Push in or forward)
Mixture Full Close. (Pull out or back)
Crank engine.
Once it starts firing, put the mixture back in and pull the throttle back to idle.

Happy engine starts!

Tried that but it doesn't work unless I flood it first with full throttle, full rich mixture and pump on for at least 5-10 seconds. Seems as if the hot engine sits for more than 10 minutes it vaporizes all the fuel and too lean to start. I've had more success at that point with 1/4 inch throttle 1/2 mixture rich and pump on.

This is exactly how I do it in my IO-540-powered M20M. Works perfectly.

I have a 1969 PA 32 300 with 420 hours on engine, for the last 12 months it has been very hard starting after a flight. My MX shop checked the left Mag impulse coupler and coil at 360 hours all checked out okay. Still hard starting hot start. Mx did 500 hour AD on left and right Mag still hard starting hot start. Mx check the timing still hard starting. Mx just put new Concord battery with a new Skytec starter in, still hard starting on cold start and hot start. Final Mx check the coil cold resistance was normal no problem. Mx then heated the coil up to simulate a hot engine and the coil failed no spark. Mx put a new coil in it start everytime hot or cold. This has been a nightmare to start and I was afraid to go anywhere and shutdown knowing it would not start until the engine cooled down. I hope this helps someone out there with the same problem. Amen.

Just wondering, by coil do your mean the magneto harness?

Also have the injectors checked.  The symptoms you describe can occur when one of the injectors is not closing completely, causing flooding or an overly rich mixture.

@Breveleri, the injection system on the Lycoming 540 engine is constant flow.  They don't close.


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What's the penalty for exceeding 250 kts below 10,000 ft MSL without authorization?
What if any enforcement actions by the FAA are likely for exceeding the 250 KIAS limitation below 10,000 ft MSL?

The question does might match the title. "What is the established penalty" and "how likely is it for that penalty to be enforced" are quite different.

You are penalized 15 knots and 1000 feet.

Perhaps you mean what’s the penalty for being caught exceeding 250 kts…   ;-)

@Jim, that in itself would probably violate something here, lol


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How was the problem of dive bomber scope fogging resolved during WW2?
During the first half of 1942, US Navy SBD bombers had problems with their bomb aiming scopes fogging over, which reduced the accuracy of their bombing during several early war operations. Was this resolved by better scopes, improvements to the existing scopes or replacing them with reflector sights?

Astronomers use heaters to keep their lenses from fogging up. Would not be surprised if this was also the solution.

This answer on quora to a similar question suggests it was fixed in the SBD-5 by using a reflector based sight instead..

@JuanJimenez A good telescope would not have a heater because the heater would cause turbulence that would ruin the image.

@AnonymousPhysicist You are mistaken. It's called a dew heater and they are very commonly used. https://optcorp.com/collections/dew-heaters

@JuanJimenez "Good" was a key word there.  Those are for small hobbyist telescopes.  Humidity is low at good locations: https://en.wikipedia.org/wiki/Teide_Observatory https://elt.eso.org/about/location/ https://www.maunakeaobservatories.org/maunakea-is-unique/

@AnonymousPhysicist Which explains why they are not needed in those larger telescopes. "Hobbyist" is an irrelevant term in this discussion. No one is going to spend a lot of money on a large telescope and then place it in a high humidity location, so they don't need them. That's all it is. The fact is that they do not "ruin" any image, otherwise no one would use them.

@JuanJimenez "No one is going to spend a lot of money on a large telescope and then place it in a high humidity location, so they don't need them."  Astronomers working on site for a nice telescope constantly complain about how cold it is; they do want heaters for the astronomers, but do not use them because it would, in fact, ruin the image.

@AnonymousPhysicist No they don't ruin the image, and I am done with this stupid conversation. Moderator, please move it into a discussion area.

And it's pretty close to off topic, anyway.

The fogging problems were solved by switching from a telescopic to a reflective gun sight.
From The Dauntless Dive Bomber of World War Two by Barrett Tillman, chapter 1:

The SBD-1 through the -4 were equipped with a three-power telescopic
sight, a holdover from the open-cockpit aircraft of the thirties. The
pilot looked through the tube, lining up the crosshairs on the target
while keeping a ball much like that of a turn-and-bank indicator
centered in its groove. The centered ball told him the aircraft was
level in the dive, otherwise the bomb would go off on a tangent when
released. The drawback to the telescopic sight was its tendency to fog
over in a dive from high altitude because of a sudden change of
temperature, and the resulting condensation clouded the lens. The
windscreen was also prone to fogging, and pilots said the tubular
sight was inadequate for aerial gunnery since it severely limited the
field of vision.
The answer was a reflector gunsight nearly identical to those used in fighter aircraft.
Beginning with the SBD-5, Mark VIII reflecting gunsights were installed, as were heaters for the windscreens. These measures eliminated the problem and were instrumental in improving bombing accuracy. But they were not available until 1943.
”

An image of the Mk. VIII reflecting sight mentioned, provided by the US Air Force Museum:

There is something wrong here because telescopes can be reflectors.  Both mirrors and lenses can fog.  Modern high quality mirrors are made of the same glass as high quality lenses, but with a thin metal coating.  The cause of the fogging is the heat capacity of the glass.


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Is it normal to use only one hand on the yoke while landing?

In this video taken from this question, the video shows the pilot using one hand on the yoke and one hand on something else (perhaps the throttle? I'm not a pilot). Is it normal to do landings with one hand?

Many pilots only use two fingers. It gives you much better control for small changes. When landing in smooth air you can set the power and trim then barely touch the yoke until the roundout. In cruise flight, you use rudder and aileron trim so that the airplane bascially flies itself with only tiny changes required. If the air is turbulent, you my need a tighter grip on the yoke, but even then you don’t need two hands.

@JScarry this should be an answer, comments are temporary and not for answers

What I find more suprising then what you are asking is: 1-releasing the yoke when still over 100 kts... (at 1:25) What about lateral control ? Seems to be quite windy. 2-exiting runway over 40 kt (70 km/h) (1:40). Isn't that a bit fast ?

@GCCampbell This is my opinion, not based on any data or official sources so it doesn’t qualify as an answer.

How many people do you know who, when driving a car, always keep both hands at 10 and 2 on the steering wheel like they teach in driving school?  It's not all that different, really.

@kebs; 40kts runway exit isn't that bad, lots of runways are built to facilitate

Your question isn't absurd: during a brief experience with student pilothood, I can recall I felt uneasy about guiding a C152's climbout exclusively with my weaker left hand.   But experienced pilots get used to both doing so and its necessity given the right hand's other jobs, additionally the B737 in your example has hydraulic controls anyway.

Re "How many people do you know who, when driving a car, always keep both hands at 10 and 2 on the steering wheel like they teach in driving school?" -- that's dangerous.  The car-driver system is dynamically unstable with your hands above the center of the steering wheel.  Easy to demonstrate while driving with a single hand on the wheel -- put one hand on the top of the wheel and give a sudden sideways pulse.  Then put one hand on the bottom of the wheel and give a sudden sideways pulse.  The car-driver system is clearly dynamically unstable in the former situation but not the latter.

I have to admit I'm quite surprised at the speed and amplitude of the movements of the pilot on the yoke here. It was my impression that controlling and aircraft required a much "lighter touch" on the controls, and if I had seen this sequence in a movie I would have dismissed that as completely bogus.

@jcaron an answer on the linked question points out that this is a low speed landing in gusty conditions; the control surfaces have a smaller effect and thus more amplitude and rapidity is required to counter the gusts.

It is absolutely normal, in fact it's rare for a pilot to have more than one hand on the yoke at any one time as it isn't required. Movies will often show pilots manhandling yokes with two hands, but that's just Hollywood. The only time two hands is required is when extra strength is needed on the controls, for instance a hydraulic failure.
When landing a pilot will have one hand on the yoke (or stick) and one hand on the throttle, as throttle adjustments are needed to maintain the correct glide angle and initiate a go-around if required.

The only time I can remember a movie having both hands on the yoke is when they are trying to pull up to not crash something. i.e. Air Force One, Bond movies, Die Hard

The thinking is 2 hands are better than one I suppose. Add the matrix, 2 hands on the cyclic. And Airplane of course ;)

Oh, there's Independence Day when they are flying the alien fighter.

In many many aircraft, from gliders and recreational planes to fighter jets and the whole Airbus fleet beyond the A300/A310 (meaning a very sizable chunk of the airline industry), control surfaces are moved using a stick and not a yoke; in such cases, using two hands is often not even  an option.
Also, you often need your other hand for something else (throttle in powered aircraft, brakes in gliders).

I never laid hands on a yoke, but one difference to a stick is obvious: the yoke can be turned as well as moved forwards/backwards, and with one hand it seems (from an outsider's perspective) that you could inadvertedly move it forward or backward while trying to turn it, and vice versa.

Little would preclude occasionally using one hand over the other on a center stick if it were actually needed (eg a WW2 fighter might in extreme cases see control forces of 70-90 pounds or more) though typically in modern aviation it would not be. On a sidestick that obviously doesn't work, but those are mostly fly by wire aircraft that would never have large control forces.

Why is using two hands on a center stick not an option?  I have many times gently added a second hand on the center stick of a glider to help reduce a tendency toward overcontrol or oscillations, including just yesterday.  PS saving one hand to hold a camera out the window for a selfie is not mandatory.

@rob74 If the plane is trimmed correctly, there will be resistance to moving the yoke (or stick) forward or backward. Doing so accidentally may be an issue for student pilots used to a car steering wheel that can’t move in that axis, but you adapt surprisingly quickly. It is much harder to unlearn using the wheel to turn on the ground.

@rob74: it's one of the first things you learn when you start training. Takes a bit of practice for some, but it's not really hard. After a while, you don't even think about it.

@quietflyer: the question asks specifically about landing. In a glider, your left hand would be on the brakes. Personally, in more than 900 hours in gliders I only recall using both hands  once, when a very rusty pilot was "stirring the pot" so much in the tow that I just grabbed the stick to stop him. As for your PS: in the old days of competition we had to actually take pictures with our left hand while putting the glider in a 90-degree bank. Besides I flew several hundred hours in flapped competition gliders, where the left hand was always on the flaps lever.

@rob74 Do any planes use a three-way stick where you can twist it?

@DKNguyen genuinely curious, what are you getting at here?

@KirkWoll Just that the claim of risk of cross-axis movement on a yoke but not a stick would not be true if three-axis sticks were used. I do not know if they are or not.

@DKNguyen what you have there is fodder for a whole new question. Please ask it!

1) Ohhh... the stars pattern on the co-pilot seat! 2) How in the world did that glider pilot get his arm stuck out through that tiny opening? It looks like he'd need 3 elbows!

@FreeMan Is it that significant? o.0

@DKNguyen presuming you're following up to my comment about the 3-way stick, yes, it's always good to ask new questions as a new question. Follow-up and clarification are appropriate in comments, but yours seems to be a different question, to me, at least. If you're asking about my comment on the seat fabric, I was just being snarky. ;)

@quietflyer You can't hold the Airbus flight control stick with two hands because it's not in the center of the pilot's body, it's off to one side. Which is why it's called the "side stick".

Yes, it is normal, and, in most cases, necessary. Control of he throttle is just as critical in such a low speed, low power setting environment as control of the control-surfaces. You need a hand on both. Also, using two hands on the yoke can tend to cause you to over-control the aircraft.

Pilot here.
When I learned, I was actively taught to fly one handed, and even then not 1 hand, but a couple of fingers and a thumb. A death grip on the yoke leads to over corrections.
During T&O, your right hand is usually glued to the throttle.  It only took my instructor pulling the throttle a couple of times on me during approach/takeoff to learn THAT one.
Left hand - index, middle, thumb on yoke (PTT, electric trim, etc)
Right hand - everything else - throttle, flaps, trim.

All true.  But that doesn't mean flying with the left hand alone may not initially make a student pilot feel uneasy until they become used to it, and its necessity.

@ChrisStratton, it took me a while to get comfortable landing on the centre line rather than keeping to the right of it.


text:
Lift direction at non-zero angle of attack for a swept wing, or for a straight wing in sideslip
When the direction of forces is popularly explained, they say "the lifting force is directed perpendicularly, and the drag force is parallel to the oncoming flow".

But what if it's a swept wing? In that case, which variant is more correct and why? Will the chosen variant apply to all objects affected by aerodynamic forces and, if not, what are the exceptions?

Also, if in reality the forces are directed as in the second variant, are most of the further formulas created in such a way as to obtain a mathematical model of the first variant? For example, the lift curveslope of a swept wing will be less than that of a straight wing - is this the result of a conversion between models? I would appreciate as much information as possible, because I want to have a solid idea.

Highly related: https://aviation.stackexchange.com/a/38579/34686

the direction of forces is popularly explained

Luckily enough science is not democratic: definition are not chosen by show of hands every 5 years but so that they make sense and cannot be mistaken.
When an object moves in a fluid, a fluid dynamic force arises. If we consider a 3D object moving in a 3D space, then this force can be decomposed in... well, 3 components.
One component is quite easy to choose: since a part of the fluid dynamic force is always pointing in the main direction of the flow (so called freestream or $V_{\infty}$), then one of those three components is simply aligned with the flow and termed drag.
Therefore the other two components are perpendicular in respect to the drag (i.e. in respect to the flow). How are they fixed? If the object is an aerodynamic object, then it makes sense to define one of those two perpendicular components as going from the belly to the upper surface of the body; this component is termed lift.
Finally, the third and last component points laterally from the right side to left side of the object.
So, variant 1 is the correct one.
The picture in your variant 2 is normally used to schematise how locally the airflow bends as it approaches the leading edge of  a swept wing and is used to explain the reduction in transonic drag given by swept wings in respect to a straight one. Here there's a good explanation of this phenomenon.

By "popular explanation" I just meant that the total aerodynamic force in such explanations decomposed in 2 components, which made me think that there is no third. Big misunderstanding on my part

@BierRitter: just joking  Yes there's a third one which normally is not taken into account except during manoeuvres or in case of helicopters

So, generally speaking, is it possible to say that the sideforce of a swept wing will be imperceptible at non-zero angles of attack?

@BierRitter: due to symmetry, what happens on the right wing equals and is opposite to what happens on the left one. So what happen laterally is normally zero in total. Maybe we can see to what happens on a oblique wing.

Lift is defined to be perpendicular to the flow.  Drag is defined to be in the direction of the flow.
What your intuition is missing is that these are not the only two aerodynamic forces.  There is also a third component of force -- typically called sideforce.
In many situations, the side force is zero or minimal, so we often work in the '2D' frame of just lift and drag.  Most aircraft are symmetrical -- when they fly in a symmetrical condition, the side force generated by the left half of the aircraft cancels with the side force generated by the right half of the aircraft.
Side force is only present when the something is asymmetrical.
We can resolve any force into three perpendicular components -- Lift, Drag, Sideforce or X, Y, Z, or East, North, Up.  At different times we make different choices of how to resolve a force because it makes analysis easier down the road.
Because of this, Lift, Drag, Sideforce is just one possible choice.  It is the choice defined with lift perpendicular to the flow and drag parallel to the flow.  One benefit of this choice is that sideforce is usually zero.

Excellent description!

It is not a “popular explanation”, it is the actual definition. To be more precise, there is some aerodynamic force and we call the component along the direction of airflow (relative to the wing) drag, and the component perpendicular to the flow lift. This applies in three dimensions, not just two.
Now two parallels are parallel in all views, so for drag, variant 1 is the only one possible.
For lift it is a bit more complicated though, because parallel to the flow might mean up, sideways or any oblique angle in between. For a symmetric aircraft the sideways components cancel, so the total lift will be up, but for each wing in isolation it may be slanted sideways.
But sweep does not slant it, dihedral does. Pressure always acts perpendicular to the surface, so lift is approximately perpendicular to the wing. And since horizontal swept wing is still horizontal, the lift will still be vertical. But if the wing has dihedral—is canted from horizontal laterally—then the lift will be “toed in” by about that angle.

"Pressure always acts perpendicular to the surface, so lift is approximately perpendicular to the wing."??? You are conflating the concept of the aerodynamic force at a specific point with the total aggregate vector sum of all aerodynamic forces from every single point on an airframe. "Pressure always acts perpendicular to the surface" is only true for the former, and is not even wrong for the latter.

@CharlesBretana, I am not conflating anything, just perhaps being a bit too fast. Because locally the pressure always acts perpendicular to the surface, any significant aerodynamic force must be perpendicular to some significant part of the surface. And since there is no surface facing sideways (except for a tiny bit at the tip), there cannot be any sideways force.

No, sorry you are. Saying that an aggregate, mathematical abstraction, that exists only as a defined component of another abstraction, (the vector sum of an infinite number of tiny forces all over the surface of an airframe, (which  are normal to the surface), is normal to that surface, when the surface in question is not even flat, (so exactly which piece of the surface are yiu tslking about), is not even wrong.

Or did I misunderstand what you said?

@CharlesBretana If the pressure is constant, the integral of pressure times normal vector (= the total force) is equal to average of normal vector. Since the pressure gradients around the wing are limited (the air accelerates to equalize pressure), and since normal vector of most of the surface is close to the average, that integral (total force) will still always be close to the average normal vector. With more variation possible in the forward/aft direction then sideways, since there is more surface sloped fore/aft.

… “the wing” is just a sloppy way to talk about a flat surface approximating the wing. Since most of the actual surface is close to parallel to this, the dominant force is close to normal to it. Not exactly, but can't be too much off. Because the area that is far from parallel to it is small, so it can't create large term in the overall integral.

Totally not true. Whst if the surface is a sphere? Whst about a  box? What about a regular wing at 60 degrees AOA? You can't just make blanket statenents that are only true in special cases that happen to match the conclusions you wish to conclude with and imply that they are universally true. And remember, "lift" is an abstract mathematical concept "defined", arbitrarily, as the component of the total aerodynamic force that happens to be normal to the flight path vector.

@CharlesBretana, I am not making any blanket statements, I am making statements in context of the answer, which is about wings of practical aircraft.

@CharlesBretana. I also started the answer by defining lift as orthogonal to flow direction. However that does not define its direction, only confines it to a plane. So the rest of the argument is about constraining it to a specific direction. By observing which components the total aerodynamic force may and may not have.

Jan, with all due respect, you "defining lift as orthogonal to flow direction."  then you make the incredible statement that "However that does not define its direction".  I, respectfully, cannot conceive of how to rationally respond to such an obvious contradiction.

@CharlesBretana because it is not a contradiction. In three dimensions, there is a whole plane that is orthogonal to any particular vector. So saying it's orthogonal to one is a constraint on the direction, but not a complete definition.

Jan. Thete is only one plane orthogonal to the flow direction. Secondly, a "constraint" is not a "definition*, complete or incomplete. So that statement, as Wolfgang Pauli said, is "not even wrong". Finally, what exactly are you implying by the statement that this is not a "complete definition"? And how is it relevant to your conflation of two concepts which are each applicable to completely different physical aggregate scales?

@CharlesBretana, are you still missing the fact we are talking about three-dimensional space? One plane still has two dimensions left. It we knew the total aerodynamic force, we could project it onto the plane and get the lift. But we don't know it (yet), so the definition provides us a constraint – it must be in that plane – but not complete specification – there is still infinitely many directions in the plane. The statement is not fully formal, but it is definitely right.

As I've said now multiple times, you are conflating a statement about the physics at a point (zero dimensions), to wit "Pressure always acts perpendicular to the surface", which talks about pressure, a vector, acting in 3-D space on a surface,  (a 2-D space), with a statement about an aggregate property of the entire airfoil or airframe, to wit, "... so lift is approximately perpendicular to the wing." .. and this second statement is not even wrong. Lift is not perpendicular to the wing it is, by definition , (and not for any other reason), perpendicular to the flight path.

The pressure or force ar each point on a surface is indeed normal to the surface. Of course. It is the result of the exchange of momentum between a particle of fluid bouncing off the surface in an elastic collision with the surface. Newtons third law demands this. But Lift is just an arbitrary engineering construct defined to make calculations easier. It's direction is not determined by any physics at any level, much less at the level governed what goes on at each point on the surface. It is, simply the component of all the forces pushing on the surface that is normal to the flight path.

@CharlesBretana I'm saying lift is normal to the flight path from the beginning. But there are infinitely many directions that are normal to the flight path. So which one is lift? (the one that is also orthogonal, well, approximately, to the midchord line—that's the whole point of the argument)

Jan, No, there are not an infinite number of directions normal to the flight path, there is only one. The Flight path is also a vector, defined by the acceleration that the body is experiencing. The total aerodynamic force is the vector sum of all individual forces pushing on the airframe. It is a single vector, pushing on the aircraft is a single direction. It results in a single acceleration vector, in one single direction. (How can a body accelerate in more than one direction?!)

One component of this force, the component parallel to the velocity vector, (we call it Drag) changes the magnitude of the aircraft velocity vector, and the other defined component, which is normal to the velocity, (called Lift) changes the direction of the aircraft's velocity vector.

Yes, you can foolishly define a component of the total aerodynamic force that lies to one side of the plane that is defined by the aircraft velocity and its acceleration, but that would be NOT EVEN WRONG.  No such force can possibly exist, as it would cause an acceleration in that direction, contradicting the assumption that this force you have defined is not in the same plane as the acceleration. If you did define such a force, you would also have to define an equal and opposite force on the other side that canceled out the lateral effect of the falsely defined force.

@CharlesBretana Now all three posts you made are not even wrong. E.g. flight path is a vector, but not the one defined by acceleration of the airframe. And did you notice the question shows just a wing, not a complete airframe?

Jan, where did I say that flight path is defined by the acceleration? Please. I referenced the 2-D plane that is defined by the two vectors velocity and acceleration. Any two different vectors define a 2-D plane. And a wing is part of the airframe.  Physics (fluid dynamics), not not distinquish between parts of an airframe. It applies to every point on any airframe, actually to any point on any body moving in a fluid.

Excuse me, but, in reviewing what I posted, i did mis-state one thing in the first of those three posts. I said "The Flight path is also a vector, defined by the acceleration that the body is experiencing.". That is indeed wrong. What I meant to say is that the normal force (Lift vector) is defined by the flight path and the acceleration.

With "Variant 2", the statement that "the lifting force is directed perpendicularly (to the oncoming flow), and the drag force is parallel to the oncoming flow" would be accurate from a side view, but not from a top view, and also not from a head-on view or tail-on view.
So "Variant 2" is not an accurate depiction of the definitions of the Lift and Drag vectors for a swept wing.
"Variant 2" tends to beg the question of whether or not sweeping a wing causes it to generate some aerodynamic sideforce, acting toward the wingtip.  Sideforce being a third aerodynamic force, defined as acting perpendicularly to both lift and drag.  That is really an entirely separate question.  This would have interesting consequences during sideslip, where one wing becomes "more swept" than the other wing, in relation to the actual direction of the airflow.  Generally speaking, the answer to that question appears to be "no"  -- and all-wing aircraft1, including swept-wing all-wing aircraft, are well known for generating minimal aerodynamic sideforce during a sideslip2.  But I have seen a reference to the idea that "leading-edge suction" would in fact tend to cause that effect to some degree.3

Footnotes:

By "all-wing" aircraft, we mean flying-wing aircraft lacking tails and fuselages, whose cross-sectional area as seen in a side view is low.  Such as the XB-35, XB-49, and B-2.

See for example this passage from the 35th Wilbur Wright Memorial Lecture which Jack Northrop read to the Royal Aeronautical Society on May 29, 1947:

Side force effects: All-wing airplanes, particularly those without
fins, have a very low crosswind derivative; thus a low side force
results from sideslipping motion. Some crosswind force is probably
important for precision flight, such as tight formation flying,
bombing runs, gun training maneuvers, or pursuit. This importance
arises because with low side force it becomes difficult to judge when
sideslip is taking place, as the angle of bank necessary to sustain a
steady sideslipping motion is small. This lack of side forces has been
one of the first objections of pilots and others when viewing the
XB-35. After flying in the N-9M or XB-35 the objection is removed,
except for some of the specific cases mentioned above. For the
correction of the lack of sideslip sense, a sideslip meter may be
provided for the pilot or automatic pilot, and for very long-range
aircraft there is a valuable compensating advantage in being able to
fly under conditions of asymmetrical power without appreciable
increase in drag.

Email or on-line forum statement by Stephen Morris, co-designer of Bright Star Millennium and Swift rigid-wing ultralight hang gliders.

This answer would benefit from addressing the issue of whether we are referencing the free-stream flow or the local flow.  Will hold off for a day or two so as not to appear to be trying distract from other more recent answer(s) -- !!

First of all, thank you for the tips on where to learn more. In fact, I don't really understand much about how local flow affects the wing, and that may be why my question lacks specificity.


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What would be the effects of installing pitot tubes on the engine covers?
What are the effects on airspeed measurements, if the pitot tubes were installed right on the engine covers of an aircraft instead of being on the nose.  Would the reading be false? If so, would the reading be underestimated or overestimated and why?

Related, maybe duplicate: Which air stream pressure / speed is measured by the pitot?

There is a very good reason to install pitot tubes in a place with relatively constant environmental circumstances: calibration.
Image source
When installed at around the nose of the aircraft, the pitot always have a good view of the airflow, at all angles of attack and of sideslip. These parameters do distort the pressure reading, that is why during aircraft development the prototype tows a flying pitot tube and compares measurements. These measurement sets are used for calibration.
If it was installed on the engine covers, not only the aircraft state but also the engine state would have influence on the reading. A nightmare to calibrate, and prone to unforeseen errors for instance if a fan blade breaks off and the wobbling fan causes an airstream situation that was not accounted for.
Best to use the prime location: the nose.

On a related note, all of the GA aircraft that I am familiar with have the pitot tube located about the midpoint of the wing. If you look at the calibration tables you can see they provide an accurate reading in normal flight, but give a a misleading reading when flaps and slower speeds (resulting in higher angles of attack) upset the airflow.

There was a picture that showed pressure differences along the length of an aircraft posted somewhere on this site, which showed there is a couple of good locations for the sensors where the differences are small and one is  a bit behind the nose. I have unfortunately forgotten suitable search terms that would find it.

@jjack, I think it was static. Total remains the same, doesn't it?

@jjack, local velocity changes the dynamic pressure ($p_d = ½\varrho v^2$), but $p_t = p_d + p_s = ½\varrho v^2 + p_s$ and that is just the Bernoulli's equation (without potential term as we assume horizontal flow), so it is constant! So as the velocity, and thus dynamic pressure, increase, the static pressure decreases. Around the nose the speed is lower, but around the fuselage it is higher, so there is a point in between where it matches the free flow speed and that is the best place for the probes.

@jjack, well, the local total pressure is the same everywhere, but the pitot tube can't recover it perfectly and the error depends on the dynamic pressure. Plus the pitot tube also needs the static pressure and that does change quite significantly with position.

@jjack, no, total pressure is the same everywhere, except behind the engines, at least until compressibility kicks in, because total pressure is just energy and only engines change the energy of the stream. It is dynamic and static pressure that change. But their sum does not and cannot. When the stream accelerates, increasing dynamic pressure, its static pressure decreases and vice versa.


text:
What is the longevity and replacement schedule for turbine engine blades?
I'm trying to understand how frequently hot section turbine engine blades are changed out. Is this a calculation based on flight hours, material, engine size or maybe something else?
Apologies if this is an obvious question, conducting some research for work and do not have any prior background in this area.
Thanks.

Welcome to Aviation.SE! If you haven't already, I would invite you to take a tour of the site and read the [help]. As for your question, I believe there a multiple cases that can result in turbine replacement. I don't have enough info to post an answer but I know there are regular inspections that take place and obviously if a problem was noted it would be investigated and possibly replaced. Also I believe even if they look good they are replaced after a certain period of time/number of cycles due to fatigue stress.

There are two primary modes of turbine blade failure, that can be predicted, and hence are used to determine the lives of turbine blades at which they need to be replaced.
One is called "creep". The Wikipedia article describes the basic mechanism. The life of the blade is purely a function of "time at temperature". If the blades experiences flight time above critical material temperature limits, it accumulates a portion of creep fatigue. The total (cumulative) amount of creek fatigue is tracked, and when the blade hits that limit, it is removed and replaced with new blades. The type of algorithm used to measure creep life is called Robinson's rule, and looks like this (where c1, c2 & c3 are coefficients related to the relative damage that occurs at the associated temperatures):
Total accumulated life = c1 x time above T1 deg + c2 x time above T2 deg + c3 x time above T3 deg.
Typically, each stage in the turbine will have its own life. (Each blade on the same stage is not individually monitored and lifed, but rather they are replaced as a set.)
Another failure mode is low cycle fatigue. This is caused by changes in centrifugal stress, caused by rpm changes from throttle cycles. When the pilot moves the throttle a significant amount, large changes in stress occur. These cause the blade material to behave like bending a wire in your fingers back and forth to make it break (you don't break it by just bending it once, or pulling hard on both ends). LCF lives are counted in a similar fashion to creep, but rather than counting time spent above a temperature limit, throttle cycles are counted. A throttle cycle might be defined as a change in rpm from one specific value to another.
If the engine is modern, the electronic control would count the creep and LCF lives consumed. If the engine is old, and only has a hydro mechanical control, the engine manufacturer will do an analysis on the type of flying the plane is expected to do, and determine the life limits in terms of flight hours. Periodically, they will try and confirm how the plane is actually being flown, and if the flight time before replacement needs to be shortened or can be lengthened. Generally, either the creep life or the LCF life is much shorter than the other, so only one failure mode is tracked for each engine stage.
This document gives a good description of turbine blade failure modes, and how they are monitored. I doubt you get information on actual lives, that is likely proprietary to the engine manufacturer. The only info you might find is probably for very old engines that they don't care about anymore. However, if you are lucky, you might finds some numbers burried in NTSB or other countries aircraft accident reports related to blade failure.
There are also periodic bore scope inspections, that may cause blades to be replaced for unscheduled (unpredictable) failure modes (FOD damage, coating loss, thermal distress) as also mentioned in the above document.


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Why was the Junkers Ju 87 not upgraded with a jet engine?
Given that the Stuka dive bomber was famous during ww2 for its dive bombing attack but as we all know it was easily shot down by enemy fighters because of its slow speed especially during the later war years when the Germans lost air- superiority.
A simple solution would be to replace the piston engine in the Stuka with a jet engine instead and the problem would be solved.
Why didn't the Germans thought of this solution?

Related: https://aviation.stackexchange.com/questions/21197/why-was-the-engine-of-the-ju-87-stuka-not-replaced-with-a-more-powerful-one

"simple solution"... I needed a laugh!

First, the Ju-87 was not easily shot down. It had manoeuvrability superior  to all fighters due to its lower wing loading and Junkers flaps, and it could take a lot of damage. With the rear-facing observer the Ju-87 pilot was in a much better position to evade all maneuvers of a pursuer. Only when faced with large numbers of enemy fighters could a Ju-87 not escape.

Battle damage on a Ju-87 (picture source)
But to put a jet engine into an airplane with a fixed landing gear will not make it faster than the much sleeker piston-powered designs to its days. Instead, the idea was to develop a new design with a retractable gear and a prone pilot position. This was the Henschel Hs-132, pictured below (source)


text:
In an emergency near a remote island is it better to land resulting in the airport closing or ditch in the water?
Consider a landing at a remote island (like Easter Island or perhaps one with a calm lagoon, like YLHI) in which the aircraft is likely to crash and close the airport. (For example a gear-up landing at a single-runway airport.) Under these circumstances would it be better to ditch than to attempt the landing at the airport, thus keeping the airport open for emergency aircraft?
If the above question is unanswerable: Has there been advice issued from an aviation safety authority regarding such a scenario?

Well, you're the emergency aircraft aren't you?

It's worth noting that the Chilean aviation authorities only allow one plane at a time to be en route to Easter Island (more or less) for precisely this reason.

Note: Getting fuel to remote locations like these is difficult, and therefore expensive. So any plane flying there will likely be “tankering”, that is it will already have fuel for the return trip on board. It makes it easy for it to divert if the airport gets closed for whatever reason. (Easter Island is relatively large and popular, so it probably gets decent supply by ships, but the other small places are less likely to).

What if the lagoon is already full of ditched aircraft and there isn't any more room?

In general, it's a bad decision to ditch if there's an alternative, even a gear-up landing (Sully ditched in the Hudson because the alternatives were much worse: virtually certain total loss on board, plus significant ground casualties due to dense population).  The safety of the crew, passengers (if present), and people on the ground is the first concern of a captain or PIC.  Ditching seldom ends well, and often results in 100% fatalities, because even with a perfect ditch, rescuers often can't reach the aircraft quickly.
For a sufficiently remote location that incoming flights may not be able to return or divert (like the example, Easter Island), there may be an option to land alongside the runway (potentially a better choice for gear-up anyway, in a location without a large fire crew, since grass is less likely to start a fire than tarmac), both for safety and to keep the runway open.

I believe landing on tarmac is preferred. Less chance of digging into the grass/dirt and flipping/cartwheeling/etc. after touchdown. Than the intact aircraft can just be dragged off the runway if needed.

Hazards of grass vs. tarmac likely depend on the aircraft configuration.  Engines below the wings, you're probably right.  Planes with smooth bellies (like most business jets, for instance) may or may not be better on pavement.

Also, "landing beside the runway" may involve using a taxiway as a landing surface :) (as long as ATC's cool with it, of course)

Looking at Google Earth, Airport Mataveri does not have a taxiway, so it's grass or runway

I was specifically thinking of Easter Island -- Lord Howe Island has even fewer options, with mountains close to both sides of the runway.  There, the choice might well be, ditch or crash on the runway -- but Lord Howe Island at least appears to have emergency services; Easter Island is just a strip of asphalt, no services (or was last time I saw photos).

Lord Howe has literally one police officer. Crash rescue would be provided by tourists supported by the Air Force from Sydney or Williamtown I believe.

If your aircraft is likely to close the airport you are the emergency. If another aircraft has an emergency requiring them to divert to the same airport  in the same time as you have closed the airport (vanishingly likely). You give them the exact situation you are trying to do.
Don't do something in an extremely unlikely situation because you are afraid another person might come along in an even less likely situation. They have to come along while you have closed the airport, you just have to have an emergency.

That said, it does happen.  LaMia 2933 and another airplane were both approaching Medellín with fuel issues; part of the reason the LaMia flight crashed is because they didn't announce their fuel emergency while the other aircraft did (and thus got landing priority).